Label selection for end-to-end label-switched traffic through a communications network

ABSTRACT

In a method of label selection for end-to-end transport of label switched traffic through a communications network between a source node and a destination node, a request message is launched toward the destination node from the source node. The request message includes a label list having one or more label identifiers indicative of respective corresponding labels available for use by the source node. The label list is revised, at a cross-connect service each successive hop between the source node and the destination node, based on labels available for use by each respective hop, to produce a reduced label list. The reduced label list includes label identifiers indicative of respective corresponding labels available for end-to-end transport of label switched traffic between the source node and the destination node.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is the first application filed for the present invention.

MICROFICHE APPENDIX

Not Applicable.

TECHNICAL FIELD

The present invention relates to mapping communications paths through acommunications network, and in particular to label selection forend-to-end label switched traffic through a communications network.

BACKGROUND OF THE INVENTION

Optical communications networks have recently become established as thepreferred backbone for data communications because of the high bandwidthcapacity of the optical fiber links. In the modern network space,packetized data traffic of multiple different protocols (e.g. internetprotocol, frame relay, asynchronous transfer mode, etc.) is transportedover a common network infrastructure. Each protocol provides its ownpacket (or frame) size and format standards. Additionally, someprotocols (e.g. IP) are specifically designed to allow packets havingwidely varying lengths. New routing protocols, for example themulti-protocol label switching (MPLS) protocol have been proposed tofacilitate multi-protocol traffic across a common networkinfrastructure.

Under the MPLS protocol, label switched packets (LSPs) are propagatedacross the network hop-by-hop along a path that is set up at thebeginning of a communications session. In general, the label assigned tothe LSP can be different for each hop, with the label conversion beingperformed by the node serving the respective hop. Where the network isdesigned for wave division multiplex (WDM) transport of data traffic, itis desirable to use a data channel (or wavelength) as the label assignto an LSP through a respective hop. In this case, end-to-end transportof an LSP through an MPLS path requires a change of wavelength in thenode serving each hop.

However, the modern network space is composed of a mixture of agilecross-connects (i.e. cross-connects capable of wavelength conversionfrom input to output) and non agile cross-connects (i.e. cross-connectsthat are unable to do wavelength conversion). Because MPLS normallyrequires that the labels (i.e. the wavelengths) must be changeable atevery hop, non agile cross-connects within the network cannotparticipate in an MPLS end-to-end optical path. This restriction forcesMPLS traffic to be routed around non-agile cross-connects, and serves asa barrier to the widespread implementation of MPLS.

A technique which allows non-agile cross-connects to participate in MPLSend-to-end optical paths is therefore highly desirable.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a technique whichenables an MPLS end-to-end optical path to be mapped through acommunications network, in which at least one and possibly all of thenodes participating in the path are non-agile.

A further object of the present invention is to provide a techniqueenabling an MPLS end-to-end optical path to be mapped across acommunications network using a common label (e.g. wavelength) for everyhop of the path, so that inter-hop label conversion is not necessary.

Accordingly, an aspect of the present invention provides a method oflabel selection for end-to-end transport of label switched trafficthrough a communications network between a source node and a destinationnode. A request message is launched toward the destination node from thesource node. The request message includes a label list having one ormore label identifiers indicative of respective corresponding labelsavailable for use by the source node. The label list is revised, at across-connect service each successive hop between the source node andthe destination node, based on labels available for use by eachrespective hop, to produce a reduced label list. The reduced label listincludes label identifiers indicative of respective corresponding labelsavailable for end-to-end transport of label switched traffic between thesource node and the destination node.

A further aspect of the present invention provides communicationsnetwork adapted for end-to-end transport of label switched trafficthrough the communications network. The communications network comprisesa source node, a destination node, and at least one cross-connectintermediate the source node and the destination node. The source nodeis adapted to launch a request message toward a destination node. Therequest message includes a label list having one or more labelidentifiers indicative of respective corresponding labels available foruse by the source node. Each cross-connect is adapted to revise thelabel list based on labels available for use over a successive hopserved by the cross-connect, to produce a reduced label list.

A still further aspect of the invention provides a cross-connect of acommunications network adapted for end-to-end transport of labelswitched traffic through the communications network between a sourcenode and a destination node. The cross-connect includes a labelavailability table, a buffer and a processor. The label availabilitytable includes label identifiers indicative of labels available for asuccessive hop served by the cross-connect. The buffer is adapted toreceive a request message propagated through the communications networkfrom the source node. The request message includes a label list havinglabel identifiers indicative of labels available for conveying labelswitched traffic between the source node and the cross-connect. Theprocessor is adapted to revise the label list included in the receivedrequest message, based on the label availability table, to produce areduced label list.

The communications network may be adapted for wave division multiplex(WDM) transport of label switched traffic, each label comprising arespective data transport wavelength.

The label switched traffic may include multi-protocol label switched(MPLS) traffic.

In an embodiment of the invention, the step of revising the label listcomprises, at each cross-connect serving a successive hop toward thedestination node, a step of intersecting the label list with a set oflabel identifiers indicative of labels available for use over the hop. Arequest rejection message may be sent to the source node if the reducedlabel list is empty.

If the reduced label list included with the request message received bythe destination node contains at least one label identifier, then one ofthe label identifiers is selected from the reduced label list. Anend-to-end label switched path is then set up between the source nodeand the destination node using the respective label corresponding to theselected label identifier. The label identifier may be selected atrandom from the reduced label list.

The step of setting up an end-to-end label switched path may includesending a mapping message containing the selected label identifier fromthe destination node toward the source node, the mapping messageretracing the path traversed by the request message. Upon receipt of themapping message at the cross-connect serving each hop, the correspondinglabel indicated by the selected label identifier is assigned to theend-to-end label switched path, if the label is still available for useby the hop.

A mapping failure message may be sent to the destination node if thelabel corresponding to the selected label identifier is not availablefor use by the hop. In this case, upon receipt of the mapping failuremessage by the destination node, the reduced label list may be strippedby removing the selected label identifier. If the thereby stripped labellist is empty, a request rejection message may then be sent to thesource node. Otherwise, if the stripped label list contains at least onelabel identifier, a new label identifier may be selected from thestripped label list, and an end-to-end label switched path set upbetween the source node and the destination node using the respectivelabel corresponding to the newly selected label identifier.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention will becomeapparent from the following detailed description, taken in combinationwith the appended drawings, in which:

FIG. 1 is a block diagram illustrating an optical communications networkusable in conjunction with an embodiment of the present invention;

FIG. 2 is a block diagram schematically illustrating principal elementsof a cross-connect usable in the optical communications networkillustrated in FIG. 1; and

FIG. 3 is a message flow diagram schematically illustrating principalmessages exchanged during the set up of an end-to-end optical path inaccordance with an embodiment of the present invention.

It will be noted that throughout the appended drawings, like featuresare identified by like reference numerals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As shown in FIG. 1, an optical network 2 usable in conjunction with thepresent invention generally comprises a plurality of cross-connects 4(four are shown in FIG. 1) interconnected by fiber optic links 6.Communications devices 8, for example end user personal computers (PCs)or local area network (LAN) servers may be connected to the opticalnetwork 2 via one or more edge nodes (or access points) 10. The opticalnetwork 2 may also be connected to one or more federated networks 12,for example an asynchronous transfer mode (ATM) or an internet protocol(IP) network, through a respective gateway 14. Within the opticalnetwork 2, each of the cross-connects 4 is configured for wave divisionmultiplex (WDM) and/or dense wave division multiplex (DWDM) transport ofpacket data traffic as will be described in greater detail below.

The present invention provides a technique for label selection for anMPLS end-to-end optical path 16 across the communications network 2between a source node 10 a and a destination node 10 b via one or moreintervening cross connects 4. The path 16 is divided into hops 18, eachof which is served by a respective node (e.g., the source node 10 a of across-connect 4) connected at the up-stream end of the hop 18. In a WDM(or DWDM) environment, the label assigned to each label switched packet(LSP), for each hop 18, is the channel (wavelength) used for conveyingthe LSP through the hop. In other words, the label designates thewavelength on which the path 16 traverses the hop 18. In the exampleillustrated in FIGS. 1 and 3, the source and destination nodes 10 a and10 bare located at respective edge nodes, and two interveningcross-connects 4 a and 4 b are incorporated into the path 16. One orboth of the cross connects 4 a, b may be non-agile, so that label (thuswavelength) conversion between hops (i.e. within a cross-connect 4) maynot be possible. Thus in accordance with the present invention, a commonlabel is selected such that the label switched end-to-end optical path16 using the common label (wavelength) can be mapped across thecommunications network 2.

FIG. 2 is a block diagram schematically illustrating the principalelements of a cross-connect 4 of the optical network. As shown in FIG.2, the cross-connect 4 includes at least one input port 20; at least oneoutput port 22; a switch 24 (which may be an optical space switch)capable of providing a signal path between respective input and outputports 20, 22; a controller 26 for controlling operation of thecross-connect 4; and a label availability table 28, which may beco-resident with the cross-connect 4 or located at a remote site andaccessible by the controller 26. The controller 26 can be provided witha buffer (not shown) for temporarily storing information.

In the embodiment of FIG. 2 the cross-connect 4 is provided with twoinput ports 20 a and 20 b and two output ports 22 a and 22 b. However,it will be appreciated that a greater or smaller number of ports may beprovided in the cross-connect 4, and/or the cross-connect 4 may beprovided with a plurality of ports of which one or more may remainunused. It will also be appreciated that the ports of the cross-connect4 may be suitably designed and connected to handle bi-directional datatraffic. However, in order to simplify the present description, and aidunderstanding of the invention, the embodiment of FIG. 2 is providedwith unidirectional ports, two of which (input ports 20 a and 20 b) areconnected to handle inbound LSPs, and the other two (output ports 22 aand 22 b) are connected to handle outbound LSPs.

The label availability table 28 dynamically stores informationconcerning the availability of each label which can be used fortransporting LSPs over hops 18 immediately downstream of thecross-connect 4. Thus for each output port-22, the label availabilitytable 28 contains a list of label identifiers corresponding to eachlabel (i.e. transmission wavelength), and an associated availabilityflag. The availability flag may conveniently be a binary value, in whicha “1” indicates that the respective label indicated by the correspondinglabel identifier is available for use, and a “0” indicates that therespective label is not available for use. The label availability table28 can be dynamically maintained by updating the availability flag aslabel switched paths 16 are set up and released. In the embodimentillustrated in FIGS. 2 and 3, four labels (indicated by respective labelidentifiers λ₁ through λ₄) may by used for label switched traffic.Clearly, more, or fewer labels may be used, as desired.

Operation of the cross connect 4, and the method of the presentinvention, is described in greater detail below with reference to FIG.3.

As shown in FIG. 3, the source node 10 a is provided with a respectivelabel availability table 30 which is closely similar to that of each ofthe cross-connects 4 a and 4 b involved in the path 16. In the exampleillustrated in FIG. 3, the label availability table 30 of the sourcenode 10 a indicates that all possible labels (λ₁ through λ₄) areavailable for use on the first hop 18 a of the path 16 (i.e. between thesource node 10 b and the first cross-connect 4 a). The labelavailability table 28 a associated with the first cross-connect 4 aindicates that, of the possible labels λ₁ through λ₄, only the firstthree labels (λ₁ through λ₃) are available for use on the second hop 18b of the path 16 (i.e. between the first and second cross-connects 4 aand 4 b). Finally, the label availability table 28 b associated with thesecond cross-connect 4 b indicates that, of the possible labels λ₁through λ₄, only the first and third labels (i.e. λ₁ and λ₃) areavailable for use on the third hop 18 c of the path 16 (i.e. between thesecond cross-connect 4 b and the destination node 10 b).

In order to select a label and set up the end-to-end path 16, the sourcenode 10 a prepares a request message R (step 100) which includes a labellist 32 identifying each of the labels which are available for use bythe source node 10 a for transporting LSPs over the first hop 18 a ofthe path 16. Conveniently, the label list 32 can be prepared byextracting each label identifier from the label availability table 30for which the associated availability flag is set to “1”. The requestmessage R including the label list 32 is then launched over a link 6(step 102) towards the first cross-connect 4 a en-route to thedestination node 10 b.

Upon receipt of the request message R, the first cross-connect 4 aextracts the label list 32 and compares it with its associated labelavailability table 28 a (step 104). The first cross-connect 4 b performsthis comparison operation by calculating the intersection between theset of labels identified in the label list 32 and the set of labelsavailable for the next hop 18 b of the path 16, as indicated by thelabel availability table 28 a. The intersection set resulting from thisanalysis is the set of label identifiers corresponding to labels whichare available for use over the first two hops 18 a, b of the path 16,this intersection set is then inserted into the request message R as areduced label list 34, and the request message R is forwarded on to thesecond cross-connect 4 b (step 106).

Upon receipt of the request message R including the reduced label list34, the second cross-connect 4 b extracts the reduced label list 34 andcompares it with its associated label availability table 28 b (step 108)in an identical manner to that described above with respect to the firstcross-connect 4 a. The resulting intersection set includes each labelidentifier corresponding to labels which are available for use over allthree hops 18 a-18 c of the path 16. This intersection set is insertedinto the request message R as a further reduced label list 34 b, andforwarded to the destination node 10 b (step 110).

It will be appreciated that this process of receiving the requestmessage R; extracting the label list, and calculating the intersectionof the label list and the set of available label identifiers in anassociated label availability table 28 can be repeated (over any numberof hops 18) until the reduced label list is empty or the request message(containing a non-empty label list) is forwarded to the destination node10 b. At any cross-connect 4 if the reduced label list is found to beempty, then a request rejection message (not shown) can be generated andsent back to the source node 10 a to indicate that no labels areavailable for the end-to-end path 16).

Returning now to FIG. 3, upon receipt of the request message R, thedestination node 10 b extracts the reduced label list 34 b, anddetermines whether or not it contains any label identifiers (step 112).In the present example, the reduced label list 34 b contains two labelidentifiers, namely λ₁ and λ₃. The destination node 10 b then selectsone of the label identifiers (e.g. by random selection), and launches amapping message M (step 114) including the selected label identifier λ₁back to the source node 10 a, and retracing the route followed by therequest message R.

As the mapping message M propagates hop-by-hop towards the source node10 a, the label corresponding to the selected label identifier λ₁ isassigned by each cross-connect 4 to the end-to-end path 16 and thecorresponding entry in the respective label availability table 28 isupdated. Thus in the example illustrated in FIG. 3, the mapping messageM including the selected label identifier λ₁ is forwarded by thedestination node 10 b over the third hop 18 c. Upon receipt of themapping message M, the second cross-connect 4 b extracts the selectedlabel identifier λ₁; assigns the corresponding label to the end-to-endpath 16; and updates its label availability table 28 b to indicate thatthe label is no longer available (step 116). The mapping message Mincluding the selected label identifier λ₁ is then forwarded over thesecond hop 18 b to the first cross-connect 4 a (step 118).

It will be appreciated that it is possible that in the period of timebetween reception of the request message R propagated from the sourcenode 10 a, and subsequent reception of the mapping message M propagatedfrom the destination node 10 b, a cross-connect 4 may have receivedanother mapping message in respect of a different path and including thesame label identifier. In this case, the label corresponding to theselected label identifier λ₁ will no longer be available, uponsubsequent arrival of the mapping message M propagated from thedestination node 10 b. This situation is illustrated in FIG. 3. Thusupon receipt of the mapping message M, the first cross-connect 4 aextracts the selected label identifier λ₁, and attempts to assign thecorresponding label to the path 16. However, the corresponding label hasalready been assigned to another path, and is therefore no-longeravailable. In this circumstance, the first cross-connect 4 a prepares amapping failure message MF including the selected label identifier λ₁and indicating that the corresponding label is no longer available foruse (step 120). The mapping failure message is then forwarded (steps122-126) hop-by-hop back to the destination node 10 b, so that resourcesalready assigned to the path 16 en-route to the destination node 10 bcan be released.

Upon receipt of the mapping failure message MF including the selectedlabel identifier λ₁, the destination node 10 b extracts the labelidentifier λ₁ and strips it from the reduced label list 34 b (step 128)previously received with the request message R. If the thereby strippedlabel list 34 c is empty, then the destination node 10 b forwards arequest rejection message (not shown) back to the source node 10 aindicating that there are no labels available for the end-to-end path16. Otherwise, the destination node 10 b selects (e.g. at random)another label identifier from the stripped label list 34 c (step 130),and attempts to set up the end-to-end path 16 by launching a new mappingmessage M′ including the newly selected label identifier toward thesource node 10 a and retracing the route followed by the request messageR. In the present example, the stripped label list contains a singlelabel identifier (λ₃), and this label identifier is inserted into thenew mapping message M′ which is launched towards the source node 10 a(steps 132, 136 and 140). At each cross-connect 4, resources areassigned to the path 16 (steps 134 and 138) and the associated labelavailability table 28 updated. Arrival of the mapping message at thesource node 10 a (at step 142) including the newly selected labelidentifier λ₃ indicates completion of the end-to-end path 16 through thecommunications network 2.

Thus the present invention provides a simple means by which anend-to-end label switch path having a common label (in the presentexample, the label indicated by label identifier λ₃) for each hop of thepath is set up. Consequently, non-agile cross-connects can participatein the end-to-end path, because it is not necessary to performwavelength conversions between hops.

The embodiment(s) of the invention described above is(are) intended tobe exemplary only. The scope of the invention is therefore intended tobe limited solely by the scope of the appended claims.

1. A method of label selection for end-to-end transport of labelswitched traffic through a communications network between a source nodeand a destination node, the method comprising the steps of: a) launchinga request message toward the destination node from the source node, therequest message including label availability indication indicative oflabels available for use by the source node; b) extracting the labelavailability indication from the request message; and c) comparing thelabel availability indication with a label availability table indicativeof labels available for a successive hop between the source node and thedestination node, to produce an end-to-end label availabilityindication; whereby the end-to-end label availability indication isindicative of respective corresponding labels available for end-to-endtransport of label switched traffic between the source node and thedestination node; wherein the communications network uses wave divisionmultiplexing (WDM) for transport of said label switched traffic, thelabel switched traffic being associated with labels for use in switchingthe traffic between the source node and the destination node, each labelidentifying a respective data transport wavelength.
 2. A method asclaimed in claim 1, wherein the label switched traffic includesmulti-protocol label switched (MPLS) traffic.
 3. A method as claimed inclaim 1 wherein said comparing further comprises: at each successive hoptoward the destination node, intersecting the label availabilityindication with the label availability table.
 4. A method as claimed inclaim 3, further comprising: at each successive hop toward thedestination node, sending a request rejection message to the source nodeif the end-to-end label availability indication is empty.
 5. A method asclaimed in claim 1, further comprising: if the end-to-end labelavailability indication included with the request message received bythe destination node contains at least one label identifier, a)selecting one of the label identifiers from the end-to-end labelavailability indication; and b) setting up an end-to-end label switchedpath between the source node and the destination node using therespective label corresponding to the selected label identifier.
 6. Amethod as claimed in claim 5, wherein the one of the label identifiersis selected at random.
 7. A method as claimed in claim 5, wherein saidsetting up further comprises: sending a mapping message containing theselected label identifier from the destination node toward the sourcenode, the mapping message retracting the path traversed by the requestmessage.
 8. A method as claimed in claim 7, further comprising uponreceipt of the mapping message at each hop, assigning the label to theend-to-end label switched path if the label corresponding to theselected label identifier is still available for use by the hop.
 9. Amethod as claimed in claim 7, further comprising: upon receipt of themapping message at each hop, sending a mapping failure message to thedestination node if the label corresponding to the selected labelidentifier is not available for use by the hop.
 10. A method as claimedin claim 9, further comprising: upon receipt of the mapping failuremessage by destination node, a) revising the end-to-end labelavailability indication by removing the selected label identifier; b) ifthe end-to-end label availability indication is empty, sending a requestrejection message to the source node; and c) if the end-to-end labelavailability indication contains at least one label identifier: i)selecting a new label identifier from the end-to-end label availabilityindication; and ii) setting up an end-to-end label switched path betweenthe source node and the destination node using the respective labelcorresponding to the selected label identifier.
 11. A communicationsnetwork for label selection for end-to-end transport of label switchedtraffic though the communications network, the communications networkcomprising: a) a source node adapted to launch a request message towarda destination node, the request message including a label availabilityindication indicative of labels available for use by the source node,wherein the label availability indication is configured to be extractedfrom the request message; and b) a hop intermediate between the sourcenode and the destination node and configured to serve an intermediatenode between the source node and the destination node, wherein theintermediate node is adapted to compare the label availabilityindication with a label availability table indicative of labelsavailable for the hop, to produce an end-to-end label availabilityindication; wherein the communications network uses wave divisionmultiplexing (WDM) for transport of said label switched traffic, thelabel switched traffic being associated with labels for use in switchingthe traffic between the source node and the destination node, each labelidentifying a respective data transport wavelength.
 12. A communicationsnetwork as claimed in claim 11, wherein the label switched trafficincludes multi-protocol label switched (MPLS) traffic.
 13. Acommunications network as claimed in claim 11, wherein the intermediatenode is further adapted to compare the label availability indication byintersecting the label availability indication with the labelavailability table.
 14. A communications network as claimed in claim 13,wherein the intermediate node is further adapted to send a requestrejection message to the source node if the end-to-end labelavailability indication is empty.
 15. A communications network asclaimed in claim 11, wherein, upon receipt of the request message, ifthe end-to-end label availability indication contains at least one labelidentifier, the destination node is adapted to: a) select one of thelabel identifier from the end-to-end label availability indication; andb) set up an end-to-end label switched path between the source node andthe destination node using the respective label corresponding to theselected label identifier.
 16. A communications network as claimed inclaim 15, wherein the destination node is adapted to set up anend-to-end label switched path by sending a mapping message containingthe selected label identifier toward the source node, the mappingmessage retracing the path traversed by the request message.
 17. Acommunications network as claimed in claim 16, wherein the intermediatenode is responsive to reception of the mapping message to assign thelabel corresponding to the selected label identifier to the end-to-endlabel switched path if the label is still available for use by the hop.18. A communications network as claimed in claim 16, wherein theintermediate node is responsive to reception of the mapping message tosend a mapping failure message to the destination node if the labelcorresponding to the selected label identifier is not available for useby the hop.
 19. A communications network as claimed in claim 18, whereinthe destination node is responsive to reception of the mapping failuremessage to: a) revise the end-to-end label availability indication byremoving the selected label identifier; b) if the end-to-end labelavailability indication is empty, send a request rejection message tothe source node; and c) if the end-to-end label availability indicationcontains at least one label identifier: i) select a new label identifierfrom the end-to-end label availability indication; and ii) set up anend-to-end label switched path between the source node and thedestination node using the respective label corresponding to theselected label identifier.
 20. An intermediate node of a communicationsnetwork adapted for end-to-end transport of label switched trafficthrough the communications network between a source node and adestination node, the intermediate node comprising: a) a labelavailability table indicative of labels available for conveying labelswitched traffic through a respective communications link connected tothe intermediate node; b) a buffer adapted to receive a request messagepropagated through the communications network from the source node, therequest message including a label availability indicator indicative oflabels available for conveying label switched traffic between the sourcenode and the intermediate node; c) a processor adapted to extract thelabel availability indication from the request message and compare theextracted label availability indication with the label availabilitytable indicative of labels available for the respective communicationslink to produce an intermediate label availability indication; andwherein the communications network uses wave division multiplexing (WDM)for transport of said label switched traffic, the label switched trafficbeing associated with labels for use in switching the traffic betweenthe source node and the destination node, each label identifying arespective data transport wavelength.
 21. An intermediate node asclaimed in claim 20, wherein the processor is adapted to compare therequest message by intersecting the label availability indicator withthe label availability table.
 22. An intermediate node as claimed inclaim 20, further comprising means for assigning a label to anend-to-end path between the source node and the destination node inresponse to a mapping message received from the destination node, themapping message containing a label identifier corresponding to thelabel.
 23. An intermediate node as claimed in claim 20, furthercomprising means for sending a mapping failure message to thedestination node in response to a mapping message received from thedestination node containing a label identifier corresponding to a labelthat is not available for converting label switched traffic.
 24. Anintermediate node as claimed in claim 20, wherein the intermediate labelavailability indication includes available labels for conveying labelswitched traffic, and the intermediate node further comprising means forsending a request rejection message to the source node if theintermediate label availability indication is empty.